In modern circuits and electronic applications, capacitors are indispensable passive components for smoothing and filtering power lines and bypassing signals. In AC/DC EMC filtering applications, the two special types of X and Y capacitors used to filter AC power supply noise are generally referred to as "safety capacitors".
This article will explain how to use X or Y capacitors and why these safety capacitors are crucial in modern circuit design.
What is an X capacitor?
X-capacitors, often referred to as "line-to-line" or "crossover" capacitors, are used to reduce EMI/RFI caused by differential-mode noise from AC power supplies. X-capacitors are connected between the live and neutral wires to filter conducted interference, voltage surges, and the negative effects of voltage transients. X-capacitors provide a clean AC signal to the circuit and are therefore susceptible to all AC line variations; exceeding the capacitor's voltage or power threshold can be dangerous. Due to their special design, the failure mode of X-capacitors is short-circuit, triggering a circuit breaker or fuse to trip under overstress. Without overcurrent protection devices installed or failing to react, X-capacitors pose a significant fire hazard.
What is a Y capacitor?
Y capacitors, often called "line-to-ground" or "bypass" capacitors, are connected between the live wire and the ground wire to handle EMI/RF caused by common-mode noise. Y capacitors are also susceptible to AC line variations due to conducted interference, overvoltage surges, and voltage transients. Exceeding the capacitor's threshold can lead to capacitor failure and pose a hazard. Unlike X capacitors, Y capacitors are specifically designed for open-circuit failure, meaning the circuit can be affected by unfiltered AC power. Y capacitors pose a lower risk of fire.
Safety capacitor classification
X capacitor (wire to wire)
Y capacitor (line to ground)
Image source: All About Circuits
Safety capacitor classification
Like many safety-critical components, safety capacitors are characterized by their capacitance and threshold values using various standards and corresponding classifications. There are many standards defining safety capacitors, such as IEC 60384-14, UL 1414, UL 1283, CAN/CSA C22.2 No.1, and CAN/CSA 384-14. The most common standard, IEC 60384-14, defines the peak pulse voltage for the safety levels of X and Y capacitors.
According to IEC 60384-14, X capacitors are classified as follows:
- Category X3 – Value pulse voltage less than or equal to 1.2kV
- Category X2 – Peak pulse voltage less than or equal to 2.5kV
- Category X1 – Peak pulse voltage between 2.5kV and less than or equal to 4.0kV
According to IEC 60384-14, Y capacitors are classified as follows:
- Category Y4 – Rated voltage less than 150VAC
- Category Y3 – Rated voltage less than or equal to 150VAC to 250VAC
- Category Y2 – Rated voltage less than or equal to 150VAC to 300VAC
- Category Y1 – Rated voltage less than or equal to 500VAC
Types of X and Y capacitors
X and Y safety capacitors filter AC signals and reduce EMI, therefore they are directly connected to hazardous AC power supply voltages and must be certified as "safety capacitors" to ensure safe operation under these conditions. Various types of safety capacitors are used in safety filtering circuits. While applications extend beyond this, surface-mount ceramic and ceramic disc safety capacitors are commonly used for interference suppression and AC line filtering applications, while film capacitors are typically used to attenuate conducted emissions.
Both ceramic and film capacitors can be used as X or Y capacitors, but factors such as capacitor shape and characteristics make one type of capacitor more suitable for certain applications. Ceramic capacitors are commonly used in antenna coupling, primary and secondary coupling such as in switching power supplies, and for suppressing line interference such as in motor control, relays, high-speed switching control of switching power supplies and inverters. Film capacitors have self-healing properties, making them ideal for capacitive power supplies, meters, automotive applications, and harsh environmental conditions.
in conclusion
X and Y capacitors filter AC power lines to reduce EMI and RFI in sensitive electronic applications. X and Y capacitors are classified and have varying capabilities to withstand continuous AC power, peak pulse voltages, and voltage transients. Regardless of the application, safety capacitors are essential to power signal-sensitive electronic equipment while reducing the risk of fire or electric shock.
